1. Field of the Invention
This invention pertains to electrical controls, and more particularly to apparatus that utilizes magnetic fields to control electrical circuits.
2. Description of the Prior Art
It has long been known that a magnetic field exists around an electrical conductor, and that the strength of the magnetic field is proportional to the amount of current in the conductor. A portion of the magnetic flux can be captured in a magnetic core that surrounds the conductor. The core can be continuous, or it can be discontinuous such that an air gap exists between two ends of the core.
The magnetic flux surrounding a current carrying conductor is frequently used to control electrical circuits. For that purpose, a magnetic reed switch or other electrical device may be placed in the air gap of a discontinuous core. When the current in the conductor, and thus the amount of magnetic flux in the core and air gap, reaches a predetermined value, the flux acts to magnetize the switch and cause it to close. The switch may be incorporated into any of a great number of circuits for controlling components based on the state of the switch.
An exemplary application of a magnetic field operated switch is weld current relay Model No. 007 036 manufactured by Miller Electric Manufacturing Company of Appleton, Wis. FIGS. 1 and 2 show the relevant components of this prior weld current relay, which is indicated by reference numeral 1. The weld current relay 1 is made with a core 3 of magnetically permeable material, such as soft iron. The core 3 has spaced apart and overlapping ends 5. The spaced apart ends 5 form an air gap between them. The ends 5 are tapered, as shown at reference numerals 7. A tube 9 of non-magnetic material, such as copper, is secured to the core in the air gap between the two core ends. Inside the tube 9 is a magnetic reed switch 11. The magnetic reed switch 11 is composed of a glass bulb 13 and a pair of contacts 15 and 17 embedded in the wall of the glass bulb, as is known in the art. The contacts 15 and 17 are connected to two wires 23 and 25 that lead to a control device typically represented by block 27. The control device 27, in turn, controls various components, typically represented by block 29, of a welding system.
An electrical cable 31 passes through the core 3. When there is no current in the conductor 31, there is no magnetic flux in the core or in the magnetic reed switch 11, and the switch contacts 15 and 17 are open. When current passes through the conductor 31, magnetic flux is induced within the core 3. The flux passes between the core ends or poles 5, where it is concentrated by the tapers 7. When the flux reaches a predetermined level, it causes the contacts 15 and 17 to close. The closed contacts activate an appropriate portion of the control device 27 to control the components 29. For example, the control 27 may control such welding system components as valves that release protective gases around a welding arc and mechanisms that feed welding wires, such that gas flows or weld wire is fed in response to the existence of weld current.
Equally important as the closing of the contacts 15 and 17 of the weld current relay 1 upon start up of current in the conductor 31 is the opening of the contacts upon shut off of the current. Opening of the magnetic reed switch 11 can control, for example, closure of valves that direct protective gases around a welding arc and cessation of feeding of welding wires. Because of the hysteresis of the magnetic material of the core 3, the level of the magnetic flux that will allow the contacts 15 and 17 to open on current shut off is always less than the level of the magnetic flux that will close the contacts upon current startup, as is shown in FIG. 3a. In FIG. 3a, point Ca represents the magnetic flux required to close a typical magnetic reed switch upon start up of current in a conductor, and point Oa represents the level of the flux at which the switch will reopen upon shut-off of current in the conductor.
The prior weld current relay 1 was designed to detect magnetic flux point Ca corresponding to current in the conductor 31 upon welding start up of between about 50 and 70 amps and to operate with a maximum current of approximately 500 amps. Upon shut off of the current, the hysteresis of the magnetically permeable core 3 causes the contacts 15 and 17 of the magnetic reed switch 11 to reopen at point Oa, which was at an acceptably low level of flux. Any residual magnetism in the core was not enough to prevent the magnetic reed switch contacts from properly opening when the current was zero amps.
The prior weld current relay 1 works very well, and a great number of them have been in service for several years. However, recent advances in welding technology have pointed out some limitations to the prior weld current relay. For example, the development of welding robots has led to the requirement for detecting an initial current that is substantially less than the formerly acceptable level of approximately 50 to 70 amps. That requirement is quite difficult to meet, as can be explained with reference to FIG. 4. FIG. 4 shows pertinent portions of a representative weld current relay 32 having a core 33 with first and second poles 35 and 37, respectively. A typical magnetic reed switch 39 is shown in the air gap 41 between the core poles 35 and 37. The switch has contacts 43 and 45, which are open in the absence of a sufficiently strong magnetic flux within the air gap 41. The contacts 43 and 45 are enclosed within a glass bulb 47. Contact 43 passes through the bulb 47 in the form of a magnetic wire 49 that is bent at 180 degrees such that the end 50 of the wire is proximate the bulb. Contact 45 passes through the bulb in the form of a straight wire 51.
In order for magnetic flux to close the contacts 43 and 45 of the magnetic reed switch 39, the flux must be strong enough to pass through three air gaps: the air gap 53 between the core first pole 35 and the wire 49; the air gap 55 between the two contacts 43 and 45; and the air qap 57 between the wire 51 and the core second pole 37. As mentioned, modern requirements dictate that the maximum detect point pull-in of a magnetic reed switch on increasing current in a conductor be as low as possible. For reasons to be explained shortly, it is not possible for the prior weld current relay 1 to employ a more sensitive switch in order to obtain a suitably low maximum detect point.
Another aspect of present welding practice is the use of high welding currents of 1,000 amps and more. After such a high current is turned off, the high levels of residual magnetism in the core 3 of the weld current relay 1 (FIGS. 1 and 2) may be great enough to keep the magnetic reed switch contacts 43 and 45 closed. That situation is illustrated in FIG. 3b. FIG. 3b shows that the opening point Ob of the switch contacts on decreasing current is not reached when the current falls to zero; it actually takes a negative current to decrease the residual flux sufficiently to open the switch. Consequently, the weld current relay occasionally misdetects the residual magnetism around a deenergized electrical conductor 31 following shut-off of high welding current as being the much lower level of magnetism associated with the initial welding current that provides the maximum detect point to close the switch on increasing current. As a result, the various components 29 of the welding equipment that are controlled by the welding current may continue to operate as though welding is occurring, even though in actuality welding has ceased.
The use of a higher sensitivity magnetic reed switch to close at a low level of increasing current would aggravate the problem of the switch remaining closed upon current shutdown. In other words, a higher sensitivity switch would sense a suitable maximum detect point pull-in on increasing current, but that switch would rarely, if ever, sense a suitable minimum un-detect point for drop out on decreasing current.
Another problem with prior weld current relays concerns the configuration of the magnetic reed switch wires, such as wires 49 and 51 of the typical switch 39 of FIG. 4, relative to the core ends 35 and 37. Looking at FIG. 5, a weld current relay 32' is shown in which an alternate way is used to bend the wires 49' and 51' of the magnetic reed switch 39'. The glass bulb 47' is rotated about axis 64' relative to the glass bulb 47 of the weld current relay 32 of FIG. 4, and the wire 49' of the switch 39' is therefore bent in a different configuration. The result is that the weld current relay 32' has different operating characteristics than the weld current relay 32, because the lines of magnetic flux 66 must follow different air gaps and paths between the core ends 35' and 37' compared with the core ends 35 and 37 of the weld current relay 32. This leads to inconsistent reed switch operation from one weld current relay to another.
To further aggravate the inconsistent operation of prior weld current relays, the angular displacement of the magnetic reed switch in the supporting tube 9 (FIG. 1) about its longitudinal axis, such as axis 64 of switch 32 and axis 64' of switch 32', is important. Thus, switch angular displacement introduces another variable into the performance characteristics of the weld current relay.
The wires of the magnetic reed switches of prior weld sensor relays are bent at random, and the switches are installed with random angular orientation about their longitudinal centerlines. Consequently, the performance of the prior weld sensor relays are quite inconsistent. Such inconsistent operations were rarely troublesome with prior welding equipment, but they are intolerable with modern welding robots and similar high technology equipment.